1. Field of the Invention
The invention relates to communication systems, and more particularly, to a channel interface unit for connecting a two-wire transmission path to a four-wire transmission path whereby all direct current (DC), low frequency and voice band signals typically carried by a two-wire analog transmission path are propagated along a four-wire digital transmission path.
2. Description of the Prior Art
Typically, telephone companies lease two-wire transmission lines to customers for use as dedicated communications and/or signaling paths between customer sites located at distant geographic locations. Customers, such as utilities, use such paths for transmitting telemetry; water, gas, and sewage flow rates; power grid monitoring signals; and signals for control of remote transmitters. Additionally, alarm companies use such two-wire transmission paths for alarm signaling between customer locations and the alarm company premises. Each two-wire transmission path is generally a two-wire transmission line contained within a large multiconductor cable. The dedicated two-wire transmission lines carry various signal formats including direct current (DC) and/or low frequency (&lt;200 Hz) analog signals and voice band signals (200 Hz-3.4 KHz). However, recently, to increase the number of voice and data transmissions over a single cable as well as to increase services while reducing maintenance costs, telephone companies have been converting existing multiconductor cables to fiber optic cables and other high speed transmission media which carry digital transmissions, i.e., digital transmission facilities. Such digital transmission facilities are only capable of transmitting voice band signals (200 Hz-3.4 KHz). Therefore, only customer systems that utilize voice band signals are compatible with these digital transmission facilities. Additionally, telephone companies have not been adding new dedicated metallic cables for customer use. As such, if a customer desires to expand its signaling capability, the customer must purchase new equipment capable of transmitting voice band signals through digital transmission facilities.
One specific illustrative customer use for a two-wire transmission line is to transmit alarm signals between distant customer sites and an alarm company premise. Typically, burglar and fire alarm companies lease two-wire transmission lines from telephone companies to connect alarm-monitoring equipment to alarm-detection equipment. The alarm-monitoring equipment is typically located in a monitoring office at the alarm company premises; the alarm-detection equipment is typically located at a customer premise. Usually, the customer premises and the monitoring office are geographically distant. The two-wire transmission line carries the alarm detection signaling, typically a direct current (DC) or low frequency analog signal. However, for some applications, voice band signals are also transmitted along with a DC signal.
Many burglar and fire alarm systems produce alarm signals that are simply not compatible with existing digital carrier systems, i.e., the alarm signals are typically DC or low frequency signals (out of the voice band) and digital carrier systems only respond to voice band signaling. Specifically, the alarm signals are not compatible with the digital channel units which form a typical interface between a customer and the digital carrier system. At present, there are at least fifty different alarm techniques which are utilized in signaling alarm conditions over two-wire transmission lines. Only a small number of these techniques are compatible with existing digital channel units, i.e., techniques that utilize voice band signals only. While some alarm companies have adapted their particular alarm signals to operate with existing digital channel units, this approach detrimentally requires alarm companies, producers of alarm equipment or telephone company subscribers to replace existing equipment with special equipment designed for use with each individual alarm signaling technique. Such an approach is costly and sometimes complicated.
A number of attempts have been made at developing interface equipment which provides compatibility amongst a number of alarm signaling techniques and existing digital carrier systems. In such a system, the digital carrier system contains a far end and a near end which are connected to one another by a digital transmission path, typically a T1 link. At each end of the digital transmission path is a channel bank containing channel units for formatting customer signals before transmission on the digital transmission path. To interface an alarm signal produced by a particular alarm signaling technique to the digital transmission path, a channel unit within each channel bank is replaced with interface equipment capable of formatting an alarm signal into a digital signal for transmission on a digital transmission path. Typically, the interface equipment, at a near end location, replaces an existing channel unit in the near end channel bank. In operation, the interface equipment digitizes all DC, low frequency and voice band alarm signals, formats the digitized signals into a 64 Kbps channel and inserts that channel into an appropriate time-slot within a digital carrier produced by the channel bank. At a far end location, corresponding interface equipment within a far end channel bank receives the digitized signal and converts the signal to an analog signal resembling the original DC, low frequency and voice band alarm signals that were digitized at the near end. Any response signal generated by the alarm equipment at the far end is digitized and transmitted by the interface equipment at the far end to the near end. In this manner, full duplex transmission of analog signals including low frequency, e.g., &lt;200 Hz and DC, can be achieved over a digital carrier system. Such interface equipment is disclosed in two patents, both of which are entitled "Channel Unit Interface Circuit" and are issued to F. J. Kiko (U.S. Pat. Nos. 4,993,063 issued Feb. 12, 1991 and 4,852,160 issued Jul. 25, 1991), both of which are herein incorporated by reference.
Two measures of operation of the interface equipment described above are DC accuracy, known in the art as DC leakage, and conversion delay, known in the art as effective capacitance. All possible anomalies that affect the DC accuracy of the interface equipment are included in the definition of DC leakage. Such anomalies include quantization error, digital-to-analog (D/A) and analog-to-digital (A/D) conversion errors, transmission errors and interface equipment non-linearities. To accurately reproduce DC and low frequency alarm signals, these anomalies must be minimized or eliminated. On the other hand, effective capacitance is a measure of how quickly a change in signal level at the input of the interface equipment at the near end of the transmission path is reflected at the output of the far end interface equipment and vice versa. In other words, effective capacitance is a measure of end-to-end system response time.
Many alarm techniques utilize exceedingly small signals, i.e., on the order of a few milliamps of differential change in a quiescent current in the two-wire transmission lines, to indicate an alarm condition or for remote testing the alarm detection equipment. To detect such small changes on long two-wire transmission lines, the alarm monitoring circuitry must be very sensitive to such changes. To effectively reproduce these small changes in current when using interface equipment, the DC leakage of the interface equipment must be minimized to a fraction of these changes. If the DC leakage is not appropriately minimized, the sensitivity of the alarm equipment will be compromised and false alarms may result. Present interface equipment is generally not accurate enough to reliably transmit and receive DC and low frequency signals used in conjunction with this sensitive form of alarm equipment. Thus, users of sensitive alarm equipment must continue to rely upon two-wire transmission lines to connect the alarm detection equipment to the alarm monitoring equipment. However, as two-wire transmission lines are replaced with digital transmission facilities, owners of such sensitive alarm equipment will be required to replace their existing equipment with equipment that is compatible with digital transmission facilities.
Moreover, many alarm systems expect the end-to-end response time of a link between the alarm detection equipment and the alarm monitoring equipment to be at least as fast as the response time of the same system using two-wire transmission lines. As such, interface equipment must provide an end-to-end response time that at least approaches the speed of the two-wire transmission line which the interface equipment replaces. To improve system response time, the interface equipment must function with minimal signal processing overhead. Signal processing overhead refers to the time required for a system to process information, such as synchronization and timing information, that is generally unrelated to signal reproduction per se. Present interface equipment does not respond nearly as fast as many alarm systems require. As a result, the alarm systems which require fast response time are relegated to using only two-wire transmission lines between the alarm detection equipment and the alarm monitoring equipment. Thus, a need exists for interface equipment that has relatively low DC leakage and effective capacitance such that the interface equipment is compatible with substantially all uses of two-wire transmission lines.
Additionally, to accurately reproduce the analog signals, the interface equipment must both sink and source current from/to the two-wire transmission lines. When sinking current, the current must be absorbed (dissipated as heat) by the interface equipment or, as in some interface equipment, be transferred into a channel bank power supply from which the interface equipment draws its power. Moreover, since all the channel units within a channel bank are connected to a common power bus, the current from the channel equipment may be transferred to other channel units as well as the channel bank power supply. Using the channel bank power supply or other channel units within the channel bank as a current sink can detrimentally effect the operation of the entire channel bank. In that regard, channel banks are not designed to absorb power from the interface equipment installed therein and any such demand on the channel bank can, for example, cause current limiting devices to malfunction. Such malfunction can present a safety hazard to channel bank service personnel or to the channel bank equipment itself.
To avoid causing the channel bank power supply to sink current, some prior art interface equipment absorb that current by dissipating it as heat. However, such interface equipment is exceedingly inefficient and, as such, the heat dissipation of the interface equipment raises the internal operating temperature of the channel bank and wastes power which limits the number of channel units which can be installed in the channel bank along with the interface equipment. Thus, a need exists for interface equipment that is efficient and that does not require the channel bank to sink current.
Additionally, in another application where telephone companies maintain two-wire transmission lines, a pair of two-wire transmission lines is used by a telephone company itself to communicate with and test, telephone lines connected to remotely located switching gear. However, with the advent of digital carrier systems, it would be advantageous for the telephone company to be able to propagate the test signals over a digital link rather than a metallic pair of two-wire transmission lines. As a result of using a digital link, installation or maintenance of a dedicated pair of wire lines would not be required. Thus, a need exists in the art for interface equipment to connect test equipment located at both ends of a traditional pair of two-wire transmission lines via a digital carrier system. Typically, the test equipment used is sensitive to relatively small signal changes carried by each of the two-wire transmission lines. Moreover, test signals and responses to those test signals occur at very high speeds. Therefore, to accomplish the interface between the test equipment and the digital carrier system, interface equipment having a relatively low DC leakage and a relatively low effective capacitance is absolutely necessary to preserve test accuracy. Previous attempts at such interface equipment have had limited application because of excess DC leakage and effective capacitance of the interface equipment.
Thus, a need exists in the art for interface equipment, specifically a channel interface unit, which is compatible with substantially all of the different alarm signaling techniques and other uses for two-wire transmission lines. Such a channel interface unit must have a low DC leakage and effective capacitance. Additionally, the channel unit must be adaptable to test remotely connected telephone lines connected through digital carrier systems. Also, the channel interface unit must not sink current to a channel bank power supply from which the channel interface unit receives its power or sink current to other channel units within the channel bank. Moreover, the channel interface unit should not operate inefficiently.